It is estimated that one of three Americans of European ancestry will develop a BCC during their lifetime. Although patient mortality is low, these tumors are locally invasive and may cause significant tissue destruction and morbidity, particularly because 80% of tumors occur on the head and neck. Deregulated Sonic hedgehog (HH) signaling has been implicated in various cancers and is the foremost, if not exclusive, genetic causative factor for BCC carcinogenesis. Being the best studied system, BCC's initiation, growth and progression have all been attributed to abnormal HH activation. This knowledge has led to the development of BCC mouse models, which in turn has markedly boosted our understanding of BCC and HH signaling. In the past, studies of carcinogenesis have focused primarily on the tumor cells themselves. However, accumulating evidence argues strongly that the tumor microenvironment, collectively referred to as the tumor stroma, plays crucial roles throughout different process of tumor formation. Within the multiple cell type- containing stromal complex, fibroblasts, aka cancer associated fibroblasts (CAFs), are the predominant component and actively support tumor growth. The pivotal roles of BCC CAFs have been implied by gene expression studies, but so far have not been directly addressed. This lack of data is partly due to limited success in past tumor grafting experiments, which in contrast has dramatically advanced stroma research in other cancer types. Based on our prior findings as well as on some insights from the literature, the goal of this project is to develop a simple yet robust allograft model that offers direct assessment of CAFs' influence on tumor growth, utilizing differentially marked tumor cells and CAFs derived from transgenic mice. As a proof-of- principle, we will use this model to address, at the fundamental level, 1) whether or not CAFs are required for tumor growth; 2) whether or not Ptch1+/- skin fibroblasts act as CAFs; and 3) whether or not irradiated skin fibroblasts act as CAFs. In line with the mission of NIAMS, the long-term goal of my research is to build upon our prior findings to achieve comprehensive understanding of BCC carcinogenesis and to discover new drug targets. Additionally, we will use BCC as a model system to elucidate the basic mechanisms of HH signaling, which holds great promises of developing better strategies to detect, treat and even prevent cancers in general.